1. Field of the Invention
The invention relates to the acquisition of NMR data from which both image and spectral data may be obtained from two or more nuclei simultaneously.
2. Description of Prior Art
Nuclear magnetic resonance (NMR) techniques have been used for over 30 years to acquire spectroscopic data. More recently NMR techniques have been developed which produce spatial images of an object (standard imaging). Even more recently techniques have been developed which provide both chemical shift spectroscopic and spatial information for a given nucleus (chemical shift imaging).
The most common technique for obtaining standard images is using the spin-echo or spin warp pulse sequence shown in FIG. 1. In this sequence, radio frequency (rf) excitation pulses of 90 and 180 degrees are used to excite the nuclei and generate an echo. The magnetic field gradients on during the rf pulses (Gss) perform the slice selection part of the experiment. The term gradient as used here means magnetic field gradient. The phase encoding gradient (Gpe) pulses provide for spatial encoding in one dimension and the readout gradient (Gro) pulses provide for spatial encoding in the other dimension. Note that the Gro is on during acquisition of the echo. Each of the echoes produced by repetition of this pulse sequence are acquired, stored and processed using a two dimensional Fourier Transform (2DFT). This technique will produce spatial images of an object.
The most commonly used technique for obtaining chemical shift images is shown by the pulse sequence in FIG. 2. In this experiment a 90 degree rf excitation pulse 208 is applied in the presence of Gss 202 for slice selection. The result of this excitation is a free induction decay 210 (FID). The phase encode and readout gradients may be thought of as phase encoding pulses in the x and y directions respectively. The stepped Gpex and Gpey pulses (i.e., the phase encode pulses in the x and y directions) provide for spatial encoding of the image. A three-dimensional Fourier Transform (3DFT) on the FIDs acquired by repetition of this pulse sequence produces a spatial image of the object and chemical shift spectra from each volume element (voxel) of the image. Note that the chemical shift spectroscopic information is preserved using this pulse sequence since no gradients are present during acquisition of the FID. It is also important to note that flip angles other than 90.degree. and 180.degree. can be used with the method of this invention.
It would be advantageous to acquire both a standard image (typically of H-1 which provides high resolution anatomical information) and a chemical shift image (typically of P-31 which provides an in vivo metabolite map) on patients submitted for magnetic resonance imaging exams. Lenkinski et al. and Tropp et al. both describe this type of exam with a sequential acquisition scheme shown in FIG. 3, Method 3A. They first acquire a standard image and then acquire a chemical shift image. However this method is rarely used in clinical situations because of the excessive time required to perform the two sequential studies. Imaging time, sometimes referred to as magnet time is very expensive. As a result of the high cost of magnet time, health professionals and/or their patients may forego certain diagnostic tests. It would be most advantageous if one could perform the two studies simultaneously in order to make the most efficient and cost effective use of imaging time. The reduced cost resulting from the procedure of this invention would allow health professionals to obtain more useful diagnostic information.